I already knew the expression, "to foot the bill," and there is also, "to fill the bill." I initially thought this was just a variant, but on closer examination it seems to be an altogether different idiomatic expression.

5 Answers
5

To foot the bill is to cover the costs or expenses of something or someone. One of the meanings of the verb foot is to pay for/cover. Thus, expressions such as I'll foot it, foot the expenses, foot the costs, etc, are also valid.

footverb (informal): pay (the bill) for something, especially when the bill is considered large or unreasonable.

Looks like lots of folks have mentioned how the two senses differ. I think it's worth pointing out that the word "bill" has several meanings in English, which ties into this. Consider these two meanings:

an itemized statement of money owed
for goods shipped or services
rendered; "he paid his bill and left";

vs

a list of particulars (as a playbill
or bill of fare)

In the case of "foot the bill", we're talking about a statement of money owed (in many cases, a literal statement on paper).

For "fill the bill" the meaning is more like "matches this set of requirements", which relates to the second meaning of "bill".

More generally, bill covers all sorts of documents, originally sealed (derived from Latin bulla in the sense of the seal on a document), then becoming more general, and then splitting into a variety of more specific senses, of which these are two (the legal and legislative senses would probably be the next most common, but there are a great many others).
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Jon HannaJan 17 '14 at 11:09

I'm not a native nor fluent speaker of English, but I think 'bill' used in the context means a kind of a printed notice for advertisement. Therefore, 'if sb fills the bill' can be interpreted to mean that he is cut out for the job.

Thanks for taking the time to answer, however the answers before your have sufficiently answered the question. I think your time would be better spent on our unanswered questions.
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Matt E. Эллен♦Jan 17 '14 at 12:57